The present invention relates to a through type twin high electric strength capacitor and, in particular to a capacitor for the use in the noise filter in a high power and high frequency apparatus, like a microwave oven, a broadcasting transmitter or an X-ray generator.
In a high frequency and high power apparatus operating in the VHF or UHF band, it is important to remove the noise superposed on a commercial power line. In view of this point, a noise filter, which is actually a lowpass filter, is generally inserted in a power supply line of such an apparatus.
Referring to FIGS. 1 and 2, a conventional through type twin capacitor disclosed in the U.S. Pat. No. 4,370,698 for example, has an elliptic ceramic body 1 which has a pair or through holes 2 and 3 in its thickness. On the upper surface of the ceramic body 1, there are mounted a pair of separated electrodes 4 and 5 having the corresponding through holes and, on the lower surface thereof there is mounted a common electrode 6 having the corresponding through holes. A ground conductor 7 has a rectangular plate 7c and an elevated portion 7a. The plate 7c has four through holes 7c-1 to 7c-4 for fixing the capacitor assembly to a filter housing (not shown) by means of screws. The elevated portion 7a has a pair of through holes 9 and 10 which correspond to the holes 2 and 3 of the ceramic body 1, respectively. The elevated portion 7a also has plural through holes 7b along the peripheral line thereof. The common electode 6 is mounted on the elevated portion 7a of the ground conductor 7 so that the holes 9 and 10 overlap with the holes 2 and 3, respectively, and that the small holes 7b are positioned outside the ceramic body 1. A pair of elongated through conductors 11 and 12 have tab terminals 20 and 21, which are to be coupled with receptacle terminals of an external circuit (not shown). The rods 11 and 12 are inserted into the holes 9 and 10 of the ground conductor 7, and the holes 2 and 3 of the ceramic body 1, respectively. The conductor rods 11 and 12 are covered with flexible insulating tubes 15 and 16 made of rubber for instance, respectively, in order to assure the insulation between the common electrode 6 and the rods 11 and 12. The insulation tubes 15 and 16 also act to absorb the shrinkage stress in an injected filler 17 generated upon the heating and hardening process thereof, and to thus prevent the occurrence of cracks or the like in the hardened filler 17. The rods 11 and 12 also penetrate metallic caps 13 and 14, which are mounted on the electrodes 4 and 5, respectively. The rods 11 and 12 are then soldered to the caps 13 and 14, respectively. The caps 13 and 14 also have an portion having plural small holes 13a and 14a, respectively. Under the ground conductor 7, there is attached a hollow elliptic cylindrical plastic cover 8 so that it encloses the rods 11 and 12 with the tubes 15 and 16. The lower end of the cover 8 is designed to slightly project beyond the lower ends of the tubes 15 and 16. The cover B is an essentially elliptically shaped column having a pair of parallel long linear walls 8a and 8b, and a pair of half-circular walls 8d and 8e connecting said long linear walls. The cover 8 has a bridge 8c across the top portion of the same across the centers of said parallel long linear walls 8a and 8b so that said bridge 8c separates the space in the cover 8 into two substantially circular areas.
An insulating filler 17, which is for instance epoxy resin, surrounds the ceramic body 1, the caps 13 and 14 and the rods 11 and 12, as shown in FIG. 2. In the injecting process of the insulating filler 17, the capacitor body is covered with the cover 18, and the filler 17 is then injected in the cover 18 through the opening of the bottom of the cover 8. The insulating filler injected from the bottom passes through the small holes 7b provided on the ground conductor 7, and the small holes 13a and 14b provided on the caps 13 and 14, and thus, the inside area of the cover 18 is filled with the insulating filler 17. After the filler thus injected is hardened, the cover 18 is removed, and the twin capacitor assembly is completed. Consequently, the electrical insulation as well as the protection against oil, moisture and/or dust is assured by the injected epoxy resin.
Another conventional through type twin capacitor is illustrated in FIGS. 3 and 4 (see the Japanese Utility Model Laid-Open Publication No. 106330/85, for example). An elliptic ceramic body 31 has a pair of holes 32 and 33 in its thickness. On the upper surface of the ceramic body 31, tnere are mounted a pair of separated electrodes 34 and 35 having the corresponding through holes and, to the lower surface thereof there is attached a common electrode 36 having the corresponding through holes. A ground conductor 37 has a rectangular plate 37c as well as an elevated portion 37a. The plate 37c has four through holes 37c-1 to 37c-4 for fixing the capacitor to a filter housing (not shown). The elevated portion 37a has a generally elliptic through hole 37b. The common electrode 36 is mounted on the elevated portion 37a of the ground conductor 37. A pair of elongated through conductors 41 and 42 at the upper ends of which there are integrally formed tabs 41a and 42a, are inserted into metallic caps 39 and 40, the holes 32 and 33 and the elliptic hole 37b. The rods 41 and 42 are soldered to the metallic caps 39 and 40 which are mounted on the electodes 34 and 35, respectively. The rods 41 and 42 are also covered with flexible insulating tubes 44 and 45, respectively, for the same purpose as the tubes 15 and 16 described previously. Onto the ground conductor 37, there is attached a hollow elliptic cylindrical plastic cover 38 so that it encloses the lower portions of the rods 41 and 42 covered with the tubes 44 and 45. In this arrangement, it should be noted that the lower end of the cover 38 is designed to slightly project beyond the lower ends of the tubes 44 and 45, as shown in FIG. 4. On the ground conductor 37, there is mounted another hollow elliptic cylindrical plastic cover 43 so that it encloses the ceramic body 31, the caps 39 and 40 and the upper portions of the rods 41 and 42, as shown in FIG. 4.
Epoxy resin is injected through the openings of the lower cover 38 as well as the upper cover 43 therein. This is because the metallic caps 39 and 40 and the elevated portion 37a have no small through holes as shown in FIGS. 1 and 2, and the assembly is thereby perfectly separated into two housings. An insulating filler 46 surrounds the outside portions of the ceramic body 31 and the caps 39 and 40, and upper portions of the rods 41 and 42. Another insulating filler 47 surrounds the inside portion of the body 31 and the caps 39 and 40, and the centre portion of the rods 41 and 42 covered with by the tubes 44 and 45.
However, the conventional capacitor of FIGS. 1 and 2 has the following disadvantages. As described in the foregoing, the lower free end of the cover 8 is designed so as to slightly project beyond the lower ends of the silicon tubes 15 and 16. We found that in this arrangement, the discharge between the tabs 20, 21 and the ground conductor 7 may be initiated even when a relatively low voltage is applied to the capacitor. This discharge is notable if the capacitor is used under oily, dusty and/or moist circumstances such as a kitchen and a large amount of oil, dust and/or moisture is thus expected to be deposited on the tabs 20 and 21, the exposed bottom surface of the filler 17 and/or the cover 8.
Secondly, the filler 17 is injected through the opening of the cover 8 therebetween after the rods 11 and 12 having the tabs 20 and 21 are inserted into the holes 9 and 10, the holes 2 and 3 and the metallic caps 13 and 14, respectively. Therefore, in the filling process of the filler, there is the possibility that the insulating resin could be accidentally deposited onto the surfaces of the tabs 20 and 21. This is intended prevent the establishment of the electrical connection between the tabs and the receptacle terminals (not shown) which are attached to the tabs.
The other conventional through type twin capacitor of FIGS. 3 and 4 has the following disadvantages in addition to the same disadvantages as the capacitor of FIGS. 1 and 2. If the capacitor is used under oily, dusty and/or moist circumstances, oil, dust and/or moisture will be deposited on the exposed upper surface of the filler 46, the exposed surface of the rods 41 and 42, and the inner and outer surfaces of the upper cover 43. Especially, a large amount of oil, dust and/or moisture is apt to be deposited on the surface of the filler 46, because a recess is formed with the surface of the filler 46 and the perpendicular wall of the cover 43. This leads to the substantial decrease of the surface resistance of the path P1 from the rods 41 and 42 via the surface of the filler 46 and the inner and outer surfaces of the cover 43 to the ground conductor 37. In other words, the insulation creeping distance of the path substantially becomes short, as shown by the symbol P2 in FIG. 4 due to oil, dust and/or moisture deposited in the recess. The discharge initiating voltage depends on the degree of the insulation creeping distance. Consequently, when a large amount of oil or the like is deposited in the recess, the creeping distance or the surface resistance is directed to be decreased, and the discharge along said path is thus likely to be generated even with a relatively low voltage applied to the capacitor.
As to this disadvantage, it may be easily considered to remove the portion of the upper cover 43 upwardly projecting beyond the exposed surface of the filler 46 so that there is no recess into which oil or the like may be deposited. However, such means is not adequate, because the insulation creeping distance will be decreased by the creeping distance provided by the removed projecting portion of the upper cover 43.
Secondly, the conductive rods 41 and 42 are integrally formed with the tabs 41a and 42a, respectively. The tabs 41a and 42a are obtained by the compression molding. In this molding process, there is the possibility that some distortion is generated at the portions of the rods which are close to the tabs. The distortion of the conductive rods 41 and 42 may bring about ununiformity of the shrinkage stress in the injected filler 47. This leads to the deterioration of the heat properties of the capacitor.